submodule: bump openthread from `8f92d2d` to `c8556df` (#321)
submodule: bump openthread from `8f92d2d` to `c8556df` (#321)

Bumps [openthread](https://github.com/openthread/openthread) from `8f92d2d` to `c8556df`.
- [Commits](https://github.com/openthread/openthread/compare/8f92d2dc81243f62d6cd609956cdb0120273272d...c8556dfd6689abc2ff01e7b7d434c8c221ac32a2)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-efr32,openthread/ot-efr32,openthread/ot-efr32